The present invention relates to a photoreceptor, in particular, an electrophotographic photoreceptor.
In electrophotographic copying by the Carlson process, an electric charge layer is deposited on the surface of a photoreceptor and after exposure to form a latent electrostatic image, it is developed by a toner and the resulting visible image is transferred and fixed to a receiving sheet such as paper. Subsequently, the toner is removed from the surface of the photoreceptor and any residual charges are neutralized to erase the electrostatic image completely and condition the photoreceptor for another cycle, thereby enabling its cyclic use for a prolonged period.
For successful operation of electrophotography, the photoreceptor must satisfy various requirements not only in terms of electrophotographic characteristics such as good chargeability and sensitivity plus small dark decay but also in terms of physical properties such as long run length and high resistance to wear and moisture during cyclic use, as well as in terms of resistance to environmental conditions such as ozone generated upon corona discharging and ultraviolet radiation emitted during exposure.
Electrophotographic photoreceptors that have been commonly used in the art are inorganic products having a photosensitive layer that is based on inorganic photoconductive materials such as selenium, zinc oxide and cadmium sulfide. The use of various organic photoconductive materials as the effective component of light-sensitive layers in electrophotographic photoreceptors has been the subject of active research and development efforts in the recent years. However, the photoreceptors developed so far along this line were not completely satisfactory in terms of sensitivity and durability. In order to overcome this problem, attempts have been made to develop an organic photoreceptor of high sensitivity and durability by composing the light-sensitive layer of two different materials, one being responsible for carrier generation and the other for carrier transport. Electrophotographic photoreceptors of this "functionally separated type" provide for a great latitude in the selection of appropriate materials that exhibit the intended functions, and this enables photoreceptors having desired characteristics to be prepared fairly easily. FIG. 6 shows an electrophotographic photoreceptor of a "functionally separated type" that employs an organic photoconductive material. As shown, it is composed of an electroconductive support 1 which is successively coated with a carrier generation layer 6 and a carrier transport layer 4. The carrier generation layer 6 and the carrier transport layer 4 combine to make a light-sensitive layer 8 which is to be negatively charged. When charged negatively, the photoreceptor shown in FIG. 6 has a greater hole mobility than electron mobility and this permits the use of hole transporting materials having good characteristics, which is a definite advantage in such aspects as photosensitivity. On the other hand, there is high likelihood that carriers (holes) are injected unevenly from the conductive support, eventually causing microscopic loss or decrease of surface charges. In reversal development, this leads to the formation of image defects as black spots (i.e., black blobs) and the quality of the image obtained will deteriorate significantly. The problem of black blobs is a phenomenon unique to photoreceptors that employ organic photoconductors having high hole mobility and this is believed to have been caused by uneven injection of carriers. Thus, it has been proposed that a subbing layer be provided between the conductive support and the light-sensitive layer to block the injection of carriers as shown in FIG. 1. The subbing layer which is indicated by 7 in FIG. 1 can be formed of various materials and known examples are resins such as polyesters, polyvinyl butyral, polyvinyl formal, polyvinyl acetate, polyvinyl chloride, polyvinyl phenol and cellulose. However, the known subbing layers are not highly effective in preventing the occurrence of black blobs. further, none of the resin systems used today in subbing layers are capable of providing a blocking capability while insuring the production of highly sensitive photoreceptors. Those which have a sufficient blocking capability to prevent the occurrence of black blobs are ineffective in producing sensitive photoreceptors. On the other hand, photoreceptors having high sensitivity cannot be produced without failure to achieve satisfactory prevention of black blobs due to insufficient blocking of carrier injection.
Under these circumstances, a technique of incorporating electron releasing materials in ethylene copolymer resins has been proposed in JP-A-60-218658 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). According to this technique, an electron releasing material is incorporated in a subbing layer to form carrier traps, which are effective to some extent in blocking the injection of carriers from the electroconductive support. However, the carrier traps in the subbing layer will increase the residual potential during cyclic use of the fabricated protoreceptor. Further, the photoreceptor does not have high sensitivity.
If ordinary ethylene copolymers are used to form a subbing layer, difficulty is frequently encountered in subsequent coating of a carrier generation layer (i.e., it will not effectively wet with the as-applied subbing layer or any uneveness in the coating of a subbing layer is prone to be reproduced in a subsequently coated carrier generation layer) or it often occurs that the subbing layer does not adhere strongly to the carrier generation layer.
Semiconductor lasers which are inexpensive, small in size and which are capable of direct modulation have recently gained popularity as light sources to be used in electrophotographic processes. Gallium-aluminum-arsenic (Ga-Al-As) lightemitting devices that are used extensively as semiconductor lasers have oscillating wavelengths longer than about 750 nm. The use of laser beams is anticipated to be applicable to printers but as a matter of fact, photoreceptors having high sensitivity to light in the longer wavelength range have not yet been developed.